The Collaborative International Dictionary of English v.0.48:

Barbican \Bar"bi*can\, Barbacan \Bar"ba*can\, n. [OE. barbican,
   barbecan, F. barbacane, LL. barbacana, barbicana, of
   uncertain origin: cf. Ar. barbakh aqueduct, sewer. F.
   barbacane also means, an opening to let out water, loophole.]
   1. (Fort.) A tower or advanced work defending the entrance to
      a castle or city, as at a gate or bridge. It was often
      large and strong, having a ditch and drawbridge of its
      own.
      [1913 Webster]

   2. An opening in the wall of a fortress, through which
      missiles were discharged upon an enemy.
      [1913 Webster] Barbicanage

WordNet (r) 3.0 (2006):

barbican
    n 1: a tower that is part of a defensive structure (such as a
         castle) [syn: barbican, barbacan]

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "barbican":
   abatis, advanced work, antenna tower, balistraria, bank, banquette,
   barbed-wire entanglement, barricade, barrier, bartizan, bastion,
   battlement, belfry, bell tower, breastwork, bulwark, campanile,
   casemate, cheval-de-frise, circumvallation, colossus, column,
   contravallation, counterscarp, cupola, curtain, demibastion,
   derrick, dike, dome, drawbridge, earthwork, enclosure,
   entanglement, escarp, escarpment, fence, fieldwork, fire tower,
   fortalice, fortification, glacis, lantern, lighthouse, loophole,
   lunette, machicolation, mantelet, martello, martello tower, mast,
   merlon, minaret, monument, mound, obelisk, observation tower,
   outwork, pagoda, palisade, parados, parapet, pilaster, pillar,
   pinnacle, pole, portcullis, postern gate, pylon, pyramid, rampart,
   ravelin, redan, redoubt, sally port, scarp, sconce, shaft,
   skyscraper, spire, standpipe, steeple, stockade, stupa,
   television mast, tenaille, tope, tour, tower, turret, vallation,
   vallum, water tower, windmill tower, work



Bouvier's Law Dictionary, Revised 6th Ed (1856):

BARBICAN. An ancient word to signify a watch-tower. Barbicanage was money
given for the support of a barbican.